As it turns out, digital camera sensors are more reflective than film.

Thus, it is quite possible that the relatively intense candle light was reflecting off the sensor, then bouncing about the lens and back to the sensor, thus generating ghosts.

Likely enough that Canon has specifically optimized lens design around minimizing the impact of such internal reflections unique to digital sensors.

Amongst other digital specific optimizations, Canon has been reshaping some of the elements to reduce internal reflections off of the sensor. The technote goes there in more detail.

Now, that might not actually be what is going on here. But it warrants further investigation and comparison. Instead of candles, some high intensity LEDs should do the trick.

Fraser Speirs — another photographer with a much bigger photog-clue-stick than me — suggested that it might be the filter. And, in fact, I do have a relatively cheap, totally flat, filter on the lens. I’ll definitely add ‘filter vs. no-filter’ to the testing criteria.

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to “SLR Lenses: Seeing Ghosts”

FYI, I told my ex about this picture (he’s a serious amateur photographer) and he confirms it was the filter that did it. He says it’s a little better with an expensive, multi-coated filter; the cheap ones are the worst. But any filter can cause this and he says you can reproduce it easily to prove it.